This project will continue to investigate the development of language skills in children with Down syndrome and children who are developing typically. Children with Down syndrome exhibit deficits in language production beyond those associated with their cognitive deficits (Miller, 1987;1988; Fowler, 1990). The deficit in language production appears to increase with advances in age (Miller, 1987). Language comprehension follows more closely the development of general cognitive skills though deficits in syntactic comprehension have been documented in older children (Rosin, et.al., 1988; Hartley, 1982;1985; Rogers, 1975). The language production skills of these children show better performance on vocabulary measures than measures of syntax (Ryan, 1975; Blount, 1968; Bartel, Bryan and Keehn, 1973; Rondel, 1978a; Miller, Budde, Bashin, & LaFollette, 1987; Miller, 1988). Several investigators have recently proposed difficulty in learning syntax as the basis for the language deficits of this group. The progress of our work to date has shown that lexical skills in production are not predicted by hearing status, health status, speech motor control status, or maternal input. The proposed research will focus on the acquisition of syntax by children with Down syndrome, first investigating lexical factors associated with the transition from single to multi-work utterances, and secondly, investigating the contrasting hypotheses of specific deficit in acquiring syntax, vs the inability to perform tasks measuring syntactic production due to short term memory or retrieval deficits. The bases of these causal constructs is the possibility of language specific cognitive deficits associated with the biological bases of the syndrome. Cross-sectional studies will focus on evaluating the influence of methodological variables on measuring knowledge of syntax in comprehension and production. Longitudinal studies will address predicting performance change and individual variation relative to modeled growth trajectories over the course of the project. This work contrasts theories proposing biologically determined, language specific cognitive mechanisms, Modularity theory, Fodor, 1983; Gardner, 1983 or Constraints theory, Markman, 1989, with theories invoking environmental or physiological mechanisms, Functionalist theory, Bates and MacWhinney, 1983; Competition Model, MacWhinney, 1987; Connectionist Models, Pinker & Mehlor, 1988. Outcomes documenting consistent deficits in acquiring syntax in both comprehension and production would support the former, while outcomes of deficits in syntax in production only of fluctuating performance over the period of development studied would implicate environmental explanation. the role of lexical categorization on the acquisition of syntax will provide theoretical validation of the work on syntax (Briane, 1987).